Take a deep breath and relax. When I first got mine I was thinking the same thing. Do an oil and filter change. Change both fuel filters. Put in a new air filter.
After all that go top your tank off. If you have a known distance drive (work,mother in laws house, or hop on the interstate for 50 miles one way, something) take that drive,then figure your mileage.
When i got mine i thought that it was drinking fuel as well but it was actully getting about 23 mpg. Now that i have worked out a lot of little bugs(vacumn leaks small fuel leaks clogged oxycat) I am consistanly getting 25 mpg.

Good luck with it these are great cars.

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86 300SDL. 250,xxx on #14 Head. One eye always on temp gauge.. Cruising towards 300K